Cooperative quantum optics in dense thermal vapours

在本世纪，操纵单个量子实体(单原子、单离子、单光子)的研究活动呈爆炸式增长。这是2012年诺贝尔物理学奖的主题，参见http://www.nobelprize.org/nobel_prizes/physics/laureates/2012/.其中一个重大突破是，纠缠这一神秘的量子属性不仅被确认为爱因斯坦“远距离幽灵行动”的核心，而且还被认为是量子信息处理这一新兴话题的资源。值得注意的是，基础理论和技术突破是共生发展的。具体地说，人们正在努力利用单个量子实体的力量，如量子计算机中的量子比特，这将产生具有前所未有的能力的计算设备，利用量子系统中复杂程度的指数级。在这个提议中描述的实验中，我们将利用我们对热原子在热蒸气中相互作用的理解，产生纠缠的光子对。我们还将利用一种被称为“预兆”的现象，即对于纠缠的光子对，一个光子的存在可以通过探测器处另一个光子的存在来触发或“预告”。除了量子力学的基础测试外，纠缠光子对和预告单光子将在量子信息处理和量子密码学的新兴领域找到巨大的用途。目前正在研究的有用光子有很多可能的来源；我们的方案提供的优势包括简单性(这样可伸缩性-构建设备的关键部件的许多相同的副本-不是问题)，以及光子的特性(它们的频率(颜色)和带宽(颜色的纯度)非常适合与量子存储器和量子中继器(也基于原子系综)接口)。除了执行研究实验之外，我们计划通过在我们系的教学实验室中加入一个类似的实验来进一步推广我们的工作，这样本科生就可以实验纠缠的光子对，并测量违反贝尔不等的情况。约翰·贝尔是一位来自贝尔法斯特的物理学家，他用数学公式概括了量子力学的“怪异”程度。违反贝尔不等式不能用经典物理学来解释，但可以解释为远处的诡异行为。我们的实验结果也将被纳入调查人员为公众和学童所做的演讲中。

During this century there has been an explosion of research activity into manipulating individual quantum entities (single atoms, single ions, single photons...). This was the topic of the 2012 Nobel Prize in Physics, see http://www.nobelprize.org/nobel_prizes/physics/laureates/2012/. One of the great breakthroughs was that the mysterious quantum property of entanglement was identified not only as being at the heart of Einstein's "spooky action at a distance" but also as a resource for the emerging topic of quantum information processing. It is noteworthy that the fundamental theory and the technological breakthroughs advance in a symbiotic fashion. Specifically, there is a drive towards harnessing the powers of single quantum entities such as qubits in a quantum computer, which could yield computing devices with unprecedented power exploiting the exponential scale up of complexity in a quantum system.In the experiments described in this proposal we will produce pairs of photons which are entangled, by using our understanding of the interactions among hot atoms in a thermal vapour. We will also exploit a phenomenon known as "heralding" where for entangled photon pairs the presence of one photon can be triggered, or "heralded", by the presence of the other photon at a detector. Entangled photon pairs and heralded single photons will find great utility in the burgeoning fields of quantum information processing and quantum cryptography, in addition to fundamental tests of quantum mechanics. There are many possible sources of such useful photons which are currently being studied; the advantages offered by our scheme include simplicity (such that scalability - building many identical copies of key components of the apparatus - is not an issue), and the characteristics of the photons (their frequency (colour) and bandwidth (purity of colour)) are ideally suited to interface with quantum memories and quantum repeaters (also based on atomic ensembles).In addition to performing the research experiments we plan to disseminate our work further by incorporating a similar experiment into the teaching laboratories at our department, such that undergraduates can experiment with entangled photon pairs, and measure a violation of Bell's inequality. John Bell was a Physicist from Belfast who formulated a mathematical expression to encapsulate how "weird" quantum mechanics can be. A violation of Bell's inequality can not be explained by classical physics, but can be interpreted as spooky action at a distance. The results from our experiments will also be incorporated into talks for the public and schoolchildren given by the investigators.